1. Field of the Invention
The present invention relates to an image forming apparatus equipped with a sheet feeding apparatus that feeds a sheet.
2. Description of the Related Art
Conventionally the image forming apparatus such as a copying machine and a printer is equipped with the sheet feeding apparatus. The sheet feeding apparatus feeds the sheet cut into a predetermined size to a transfer position one by one in order to transfer a toner image, formed on a photosensitive member, onto the sheet at the transfer position.
For example, Japanese Patent Application Laid-Open No. 7-196187 discloses a sheet feeding apparatus having a configuration in which air is blown to the sheet from one end side in a conveyance direction of the sheets loaded in an accommodation box using a separating fan and the floating sheet is adsorbed to and conveyed by a conveyance belt.
However, a floatation amount depends on a material (thickness or weight) of the sheet. Therefore, there is proposed a configuration in which lifting and lowering of the tray on which the sheets are loaded are controlled such that the sheet floatation by the air blow is positioned within a predetermined range. For example, in a configuration disclosed in USP 200506068A1, position detecting unit determines whether or not the sheet floatation position is located within a predetermined range, and the tray is lifted and lowered such that the floating sheet is located within the predetermined range when the sheet floatation position is located out of the predetermined range.
There is also proposed a configuration in which a rotating speed of the separating fan is controlled such that the sheet becomes the optimum floatation amount irrespective of the material. When the sheet is thin, or when sheet is made of a light material, the control is performed such that the rotating speed of the separating fan is decreased. When the sheet is thick, or when sheet is made of a heavy material, the control is performed such that the rotating speed of the separating fan is increased. Specifically, in a configuration disclosed in Japanese Patent Application Laid Open No. 7-89625, a distance measuring sensor measures a distance from a belt surface of the conveyance belt to the floating sheet, and the air blowing quantity is controlled based on the measured distance by controlling the rotating speed of the separating fan.
There is also proposed a configuration in which the material (thickness and weight) of the sheet on the tray is inputted from an operation portion of the image forming apparatus and the air blowing quantity is uniquely determined according to the inputted material of the sheet.
In the technique disclosed in USP 200506068A1, even if the materials (thicknesses and weights) of the sheets differ from one another, the sheet is positioned within a predetermined range by the lifting and lowering of the tray. However, it cannot be determined whether or not the sheet positioned within the predetermined range becomes an optimum dealing state. As disclosed in Japanese Patent Application Laid-Open No. 7-89625, in order to obtain the optimum dealing state irrespective of the material of the sheet, the rotating speed of the separating fan is changed in each time the material (thickness and weight) of the sheet loaded on the tray is changed. However, even if the rotating speed of the separating fan is set in each sheet material to obtain the proper air blowing quantity, sometimes the proper rotating speed is not stably obtained due to a fluctuation of the fan itself, aged deterioration of fan characteristics, and voltage drop caused by bundle conductors. In this case, for example, when the sheet is thin, or when sheet is made of the light material, the rotating speed of the separating fan is faster than the target value, the sheet dealing state is not stabilized, which possibly results in a trouble such as sheet jam, skew feeding, position shift, a flaw, folded sheet, and dirt. When the sheet is thick, or when sheet is made of the heavy material, the rotating speed of the separating fan is slower than the target value, the proper air does not flow between the sheets, which possibly results in a trouble such as conveyance of overlapped sheets.
In the configuration in which the sheet material (thickness and weight) is inputted from the operation portion of the image forming apparatus, sometimes sheet material (thickness and weight) which is inputted from the operation portion differs from the material of the sheet which is actually loaded on the tray. In such cases, the rotating speed of the separating fan becomes improper value, which possibly results in the trouble such as the conveyance of overlapped sheets, the skew feeding, the position shift, the flaw, the folded sheet, and the dirt.